Process for preparation of 2,6-dimethyl naphthalene

ABSTRACT

IN THE PREPARATION OF 2,6-DIMETHYL NAPHTHALENE BY FEEDING 1,5-DIMETHYL NAPHTHALENE, 1,6-DIMETHYL NAPHTHALENE, A MIXTURE THEREOF, OR A MIXTURE CONTAINING SUCH DIMETHYL NAPHTHALENE AS THE MAIN COMPONENT TO THE ZONE OF AN ISOMERIZATION REACTION, ISOMERIZING THE DIMETHYL NAPHTHALENE IN THE PRESENCE OF AN ISOMERIZATION CATALYST, COOLING THE RESULTING ISOMERIZATION REACTION PRODUCT TO CRYSTALLIZE 2,6-DIMETHYL NAPHETHALENE, AND SEPARATING AND RECOVERING THE SAME, THE IMPROVEMENT WHEREIN 2,7-DIMETHYL NAPHTHALENE OR ITS EUTECTIC MIXTURE IS REMOVED BEFORE THE MOTHER LIQUOR REMAINING AFTER THE RECOVERY OF 2,6-DIMETHYL NAPHTHALENE FROM THE ISOMERIZATION REACTION PRODUCT IS RECYCLED TO THE ISOMERIZATION REACTION ZONE.

PROCESS FOR PREPARATION OF 2,6-DIMETHYL NAPHTHALENE s sheets-sheet 1 229mm A @mi I1 m .mi

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Unted States Patent Office Patented Mar. 19, 1974 Japan Filed Jan. 24, 1972, Ser. No. 219,994 Claims priority, application Japan, Jan. 27, 1971, 46/2,734 Int. Cl. C07c 15/24 U.S. Cl. 260-668 F Claims ABSTRACT OF THE DISCLOSURE This invention relates to a process for advantageously preparing 2,6-dimethyl naphthalene of high purity in improved yield industrially.

In the art a process has been well known for preparing 2,6-dimethyl naphthalene which comprises the steps of feeding 1,5-dirnethyl naphthalene, l,6dimethyl naphthalene, 1,6-dimethy1 naphthalene, a mixture thereof, or a mixture containing such dimethyl naphthalene as the main component to the zone of an isomerization reaction, isomerizing such dimethyl naphthalene in the presence of an isomerization catalyst, cooling the resulting isomerization reaction product to crystallize 2,6-dimethyl naphthalene, and separating and recovering the same.

In working the above process on an industrial scale, it is common to recycle the mother liquor from which the 2,6-dimethyl naphthalene has been separated, to the isomerization reaction zone. However, as such industrial operation is continued, the yield and purity of the resulting 2,6-dimethyl naphthalene are gradually reduced, though the reason for such reduction has been unknown.

2,6-dimethyl naphthalene is a useful compound which can be formed by known oxidation reactions into a Valuable starting material for preparing polyesters useful in the preparation of synthetic fibers, films, resins, etc. Accordingly, the art has demanded a process capable of preparing 2,6-dimethyl naphthalene of high purity in good yield and with high reproducibility.

Under such circumstances, research has been conducted with a view to developing a process where the above mentioned undesired reduction of the yield and purity of the product 2,6-dimethyl naphthalene is not brought about, and it has been surprisingly found that in the above recycling isomerization reaction system, 2,7-dimethyl naphthalene and/ or a eutectic mixture thereof is formed which `causes the above-mentioned undesired reduction of the yield and purity of the 2,6-dimethyl naphthalene product.

It has been well known that dimethyl naphthalene (which will sometimes be referred to simply as DMN hereinbelow) includes various isomers, and the system of shifting of the methyl group includes, for instance, the following three types:

It has also been known that no shifting occurs between the system (1) and system (2) (see, for instance, J. Org. Chem., 29, 2939, 1964).

Surprisingly it has now been found that when the isomization reaction is conducted =by the abovementioned recycling operation with use of 1,5-dimethyl naphthalene and/or 1,6-dimethyl naphthalene which has been purified to the chemically pure product and is free of any of 1,7-, 1,8- and 2,7-dimethyl naphthalenes, 2,7-dimethyl naphthalene is formed in the isomerization reaction product, though details of the mechanism of formation of 2,7- dimethyl naphthalene have not been completely elucidated. It has also been found that the so formed 2,7- DMN is not isomerized to 2,6-DM'N, as described above, but is accumulated in the recycling reaction system and causes the reduction of the yield and purity of 2,6-DMN. Further, it has been found that this 2,7-DMN formed as a by-product can be easily removed from the recycling reaction system by simple means and that the abovementioned disadvantages can be overcome by `conducting this removal operation at any optional stage after the isomerization but prior to recycle of the mother liquor to the isomerization reaction zone.

Accordingly, the primary object of this invention to provide a process for preparing 2,6-dimethyl naphthalene according to which 2,6-dimethyl naphthalene of high purity can be obtained in high yield and with good reproducibility.

Other objects and advantages of this invention will be apparent from the description given herein below.

In the process of this invention, 2,7-dimethyl naphthalene formed as a by-product is removed from the recycling isomerization system in the form of its eutectic mixture, for instance, a eutectic mixture of 2,6-DMN and 2,7-DMN or a eutectic mixture of 2,6-DMN, 2,7-DMN and 1,5-DMN, at a stage after the isomerization reaction but prior to recycling of the mother liquor to the isomerization reaction zone. Accordingly, in the process of this invention starting charges to be added to the recycling isomerization reaction system are composed mainly of DMN isomers belonging to the shifting type of In the process of this invention especially good results are obtained by employing starting charges substantially free of DMN isomers of the shifting type of Several embodiments of the removal of 2,7-dimethy1 naphthalene according to this invention are illustrated in accompanying FIGS. l and 2.

In the embodiment illustrated in FIG. 1, the isomerization reaction product coming from isomerization reaction zone 1 is cooled in crystallization zone 2 which is maintained at a temperature at which both 2,6-DMN and a eutectic mixture of 2,7-DMN crystallize or at a temperature lower than such crystallizing temperature, preferably at a temperature lower by about 5 C. than such crystallizing temperature, whereby 2,6-'DMN and the eutectic mixture of 2,7-DMN are allowed to crystallize. Then, the crystallized product is seprated from crystallizaton zone 2. Thus, the eutectic mixture of 2,7-DMN is removed from the recycling isomerization reaction system. The crystallized product is then forwarded to purification zone 3 where it is treated with an organic solvent which has a low dissolving property for 2,6-DMN but a high dissolving property for the eutectic mixture of 2,7- DMN. As a result, purified 2,6-DMN can be recovered. The mother liquor from which the purified 2,6-DMN has been separated is then condensed according to need and cooled in zone 4, and the crystallizing eutectic mixture of 2,7-DMN is removed from the mother liquor. Then the purification solvent is removed and the remaining crystallized product is recycled to the isomerization reaction zone.

In the embodiment illustrated in FIG. 1, distillation zone is provided for distilling off monoand tri-methyl naphthalenes. This zone is provided according to need in conducting the process of this invention. In the illustrated embodiment, this distillation zone 5 is provided in the recycle system prior to recycling of the mother liquor coming from the crystallization zone to the isomerization reaction zone. This distillation zone 5 may be provided, for instance, midway between zones 1 and 2. The distillation of monoand tri-methyl naphthalenes formed as by-products may be conducted at any optional stage after the isomerization reaction but prior to recycle of the mother liquor to the isomerization recation zone.

Another embodiment of removal of 2,7-DMN in the form of its eutectic mixture from the recycling isomerization reaction system is illustrated in FIG. 2.

In the embodiment illustrated in FIG. 2, the reaction product from isomerization reaction zone 1 is cooled in crystallization zone 2 at a temperature at which 2,6-DMN begins to crystallize or at a temperature lower than such temperature but higher than the temperature at which the eutectic mixture of 2,7-DMN is allowed to crystallize. The crystallized 2,6-DMN is separated and purified in purification zone 3 in the same manner as described with respect to the embodiment illustrated in FIG. 1. The mother liquor coming from crystallization zone 2 is forwarded to a second crystallization zone 2' Where it is further cooled at a temperature at which the eutectic mixture of 2,7- DMN present in the mother liquor dissolved therein is allowed to crystallize substantially or at a temperature lower than such temperature (of course, at a temperature higher than the temperature at which the entire mother liquor freezes). The crystallized eutectic mixture is removed from the recycle system. The mother liquor is then recycled to isomerization reaction zone 1 as in the embodiment illustrated in FIG. 1.

As exp-lained in the embodiment illustrated in FIG. 1, the distillation of monoand tri-methyl naphthalenes can also be performed in the embodiment illustrated in FIG. 2. In the embodiment illustrate in FIG. 2, the mother liquor in purification zone 3 is substantially free of the eutectic mixture of 2,7-DMN, and therefore, it is possible to recycle crystallized products remaining after removal of the purification solvent from such mother liquor, directly to the isomerization reaction zone.

CFI

tones such as acetone and methyl ethyl ketone, and ethers such as diethyl ether and diisopropyl ether can be exemplified. These organic solvents may be used singly, or admixtures of two or more of them can be employed.

The purification temperature may be suitably chosen depending on the kind of the purification solvent to be used, and it is preferred to select the temperature at which the solvent used has as low a dissolving property as possible for 2,6-DMN and exhibits as high a dissolving property as possible for the eutectic mixture of 2,7-DMN. Generally, temperatures ranging from 5 C. to 30 C. are adopted for the purification.

The isomerization reaction per se is well known in the art. Accordingly, a detailed description has been omitted although the isomerization is generally conducted in the following manner.

For instance, starting DMN isomers are contacted in the gas phase at a liquid space velocity of 0.2-10 v./v./hr. at 260400 C. with a solid acid catalyst such as silicaaluimina and zeolite, or they may be contacted in the liquid phase at -30 to 50 C. with a Friedel-Crafts catalyst such as boron trifiuoride and aluminum chloride.

The operation conditions in the process of this invention will now be described in more detail.

The temperature at which the isomerization product is crystallized for separation in the crystallization zone is suitably selected depending on the composition of the isomerization product. In the above-mentioned Ifirst embodiment illustrated in FIG. 1, itis preferred that the isomerization reaction product is cooled at a temperature below the temperature at which the two-component eutectic mixture of 2,6-DMN and 2,7-DMN or the three-component mixture of 2,6-DMN, 1,5-DMN and 2,7-DMN is allowed to precipitate. The precipitating temperatures T2 and T3 of the two-component and three-component eutectic mixtures can be determined based on curves illustrated in FIGS. 3 and 4.

FIG. 3 illustrates the relation between the precipitating temperatures of the two-component eutectic mixtures of 2,6-DMN and 2,7-DMN and of 2,6-DMN and 1,5-DMN and the composition of the isomerization reaction product.

FIG. 4 illustrates the relation between the precipitation temperature of the three-component eutectic mixture of 2,6-DMN, 1,5-DMN and 2,7-DMN and the composition of the isomerization reaction product in the case where 2,7-DMN precipitates as the third component.

In FIG. 3 the value of Na/(1-N2,8) is graduated on the abscissa, wherein NL designates the mole ratio of 1,5-DMN or 2,7-DMN in the isomerization product and N21; designates the mole ratio of 2,=6-DMN in the isomerization product.

In FIG. 4 the value of N23/ [l-(NZ-l-NLQ] is graduated on the abscissa, wherein Nm, Na and N15 designate mole ratios of 2,7-DMN, 2,6-DMN and 1,5-DMN in the isomerization product, respectively.

Examples of the precipitating temperatures T2 and T3 are shown in Table 1 below.

TABLE 1 DMN composition in isomerization product (mole percent) Organic solvents which have a low dissolving property for 2,6-DMN but can dissolve substantial amounts of eutectic mixtures of 2,7-DMN to be removed, are used for purification of the 2,6-DMN in the above embodiments of this invention. As such organic solvent alcohols having 1-2 carbon `atoms, hydrocarbons having 4 to 8 carbon atoms, such as butane, pentane, hexane, heptane, cyclopentane, cyclohexane, benzene, toluene and xylene, ke-

2,7DMN it is necessary to cool the isomerization reaction product at a temperature lower than 11.5 C.

In isomerization product sample No. 2, if the cooling is effected below l C., a three-component eutectc mixture of 2,6-DMN, 1,5-DMN and 2,7DMN is precipitated. Accordingly, for accomplishing the removal of 2,7DMN it is sufficient to cool the isomerization reaction product at a temperature lower than --1 C.

In isomerization product samples Nos. 3 to 5, since the content of 2,7DMN is higher, 2,7DMN is allowed to precipitate at a temperature lower than T2 in the form of a two-component eutectc mixture with 2,6-DMN.

In the above-mentioned first embodiment, the isomerization reaction product is cooled at a temperature below the lprecipitating temperature of 2,7DMN, i.e., T2 or T2, which is determined depending on the composition of the isomerization product. Based on this cooling temperature the ratio of precipitated 2,7DMN to 2,7DMN remaining in the mother liquor to be recycled to the isomerization step is determined. It is preferred that the ratio is so chosen that the amount of 2,7DMN removed from the recycle system is not smaller than the amount of 2,7DMN formed in the isomerization step.

In the above-mentioned second embodiment of this invention illustrated in FIG. 2, the temperature is maintained at a level higher than the 2,7DMN precipitating temperature T2 or T2 in crystallization zone 2. When the value of 1,5DMN/2,7DMN is greater than 1.38, namely when 1,5-DMN is allowed to precipitate in the form of the eutectic mixture, two procedures may be adopted. According to one procedure, the temperature is maintained above T2 to precipitate 2,6-DMN, and according to the other procedure the temperature is maintained between T2 and T2 so as to precipitate the eutectc mixture of 2,6-DMN and 1,5-DMN as well as 2,6-DMN. Both procedures are illustrated more specifically in Examples 3 and 2 which will be given hereinbelow. To achieve the object of this invention, namely to remove the DMN eutectc mixture containing 2,7DMN at a high concentration from the recycling isomerization reaction system at the second crystallization zone 2', the l-atter procedure is preferred.

Crystals of 2,6-DMN recovered from crystallization zone 2 contain usually the mother liquor to some extent, and therefore, it is diicult to obtain a mixture having the theoretcial composition. For instance, if the separation is conducted at a temperature exceeding T2, theoretically the separated product should be pure 2,6-DMN. However, because of accompanying mother liquor, the 2,6-DMN actually separated has a purity of about 85 to about 95%. Accordingly, a purification step is required to obtain 2,6-DMN of a higher purity. When the separation is carried out at a temperature below T2 or T2, the crystallized product comprises 1,5-DMN, 2,7DMN and the like in addition to 2,6-DMN. Accordingly, in such case a purification step should naturally be required to obtain 2,6-DMN of a high purity.

This purification is accomplished by washing or recrystallizing the crystallization product recovered from separation zone 2 with use of a solvent. In view of the object of this washing treatment, it is preferred that the treatment temperature is above the two component eutectc mixture-precipitating temperature T2. It must be noted that in this case the temperature T2 should be determined with respect to the entire system including the washing solvent.

In this invention, a DMN mixture containing 2,7DMN at a high concentration is separated in zone 4 or 2'.

In the first embodiment, the mother liquor from which 2,6-DMN has been separated at the purification step, is cooled to a temperature below T2 or T2, to thereby precipitate a two component eutectic mixture of 2,6-DMN and 2,7DMN or a three-component eutectc mixture of 2,6-DMN, 1,5-DMN and 2,7DMN. It is also possible to accomplish the precipitation of the eutectc mixture by removing a part of the solvent or conducting this solventremoving treatment in combination with the above cooling treatment.

Remaining DMN isomers are recovered from the above mother liquor and they may be recycled to the isomerization step directly or through the distillation step.

It is also possible to adopt a method comprising removing all the solvent from the mother liquor and separating the eutectc mixture from the remaining DMN mixture.

In the second embodiment, the mother liquor from which the mixture containing 2,6-DMN or the two-component eutectc mixture of 2,6-DMN and 1,5-DMN has been separated at the crystallization step, is cooled at a temperature below T2 or T2 to precipitate a 2,7DMN- containing eutectc mixture, and it is separated from the mother liquor. The mother liquor from which the 2,7- DMN-containing eutectc mixture has been separated is forwarded to distillation zone 5 where low-boiling-point and high-boiling-point by-products are removed, and then the mother liquor is recycled to the isomerization step 2.

At each step explained hereinabove, the cooling is not limited to the one-step cooling. For instance, in order to facilitate the transportation of the slurry, it is possible to lower the temperature gradually up to T2 by multistaged cooling procedures.

At the first step of the process of this invention, namely the step of isomerizing DMN isomers, a solid acid catalyst such as silica-alumina, alumina-boria or zeolite is employed. When the isomerization is conducted with use of such acid catalyst, low-boiling-point by-products such as methyl naphthalenes and high-boiling-point by-products such as trimethyl naphthalenes are contained in the isomerization reaction product. The presence of such byproducts results in lowering of the eutectc mixture-precipitating temperatures, T2 and T2. For instance, if the isomerization reaction product contains such by-products at a content of 5%, the operation temperature is lowered by more than 10 C. as compared with the case of the isomerization product free of such by-products. The lowering of the cooling temperature results in increase of expenses required for cooling and therefore, the presence of such by-products is not preferred from an economical viewpoint. Accordingly, when the isomerization reaction product contains such low-boiling-point and high-boilingpoint by-products in considerable amounts, for instance, more than 5%, it is preferred to provide distillation zone 5 between zones 2 and 3 and to forward the isomerization reaction product to the crystallizing step after removal of such by-products.

Further, if these low-boiling-point and high-boilingpoint by-products are recycled to the isomerization step,

disproportionation or trans-alkylation of methyl naphthalenes and trimethyl naphthalenes is caused to occur, and as a result DMN isomers other than 2,6-DMN, 1,6- DMN and 1,5-DMN are formed in large amounts. Accordingly, it is preferred that major portions of lowboiling-point and high-boiling-point by-products formed at the isomerization reaction are removed and then the DMN fraction is recycled to the isomerization step.

As described above, in the process of this invention the isomerization reaction is carried out by employing 1,5- DMN and/or 1,6-DMN as the starting material, 2,6- DMN is separated and recovered from the isomerization reaction product, and a mixture containing 2,7DMN in the form of a eutectic mixture is removed from the system prior to the recycle of the mother liquor to the isomerization step, whereby 2,6-DMN of high purity can be obtained at high eiciency and in excellent yield. Thus, this invention makes great industrial contributions to the art.

This invention will now be explained more specifically by referring to examples, where parts are parts by weight unless otherwise indicated.

7 EXAMPLE 1 A DMN mixture of a composition (I) indicated in Table 2 was isomerized at 290 C. at a liquid hourly space velocity of 3.0 with use of a silica-alumina catalyst 8 ing DMN mixture having the same composition as that of the starting DMN mixture (I) indicated in Table 2 were obtained. This starting mixture was isomerized under the same conditions as described above to yield a DMN mixture having the same composition as that of the which had been treated with steam under pressure. The 5 composition of the resulting reaction product is also shown leellzatlen preduel (lll) .meleated m. Table 2' in Table 2. en 3.6 parts of the mixture contalmng 2,7-DMN at 100 parts of the above reaction product were cooled e hlgh. eelltent weee not relnovd ht recycled Ito gf at C. (lower by 5 C. than the three-component euteclzse/.llglzlllen realllfm stetig lll l e .a ev e ellallnpe e tic mixture-precipitating temperature of C.), and pre- 10 07 1 remeeb regl ehlsemellzellell eye edls onli; cipitated crystals were separated to yield 39.4 parts of 27 glelfperen. dese nl e lsemrlzztg/Io lle et a DMN mixture having a composition shown in Table 2. I d teen ame tm e lllleleeyere f DMNple The so formed crystals were incorporated into 62 parts .n er .er .e prevee eeellm allen e :i thm t .le of methanol, agitated at C. to effect washing, and lsfellerlatlenfsyslel lt l .eeesseliy te le uee e Talle separated from the methanol by filtration. As a result 15 e l MN eml at the .lsemellzallen slel bele $52 29.3 parts of 2,6-DMN ha'ving a purity of 99.0% were lne e pel-eem ease .en t e lsemerlzatlen prend' f .e Obtained 1somer1zat1on 1s carried out under such condmons as will The methanol mother liquor used for washing was eallely the ebgve requlrement .the lsemellzatleneegree concentrated until its volume became 1/2 of the original ls lowered an the eyelllg. falle ls mereased Wltll the volume, and precipitated crystals were separated thereresult that the letal ylel le greatly lowered' However from. As a result 3.6 parts of a mixture having a high when 3'6 parts of erystalsuv) Obtalned byeoneentrat 2,7-DMN content, whose composition is shown in Table mg the methanol @other llquvor used for'puf-1cat10n are 2 were obtained. In order to prevent accumulation of remOVed frOm the isomerizatlOIl Cycle, @Ven 1f 2,7DMN 2,7-DMN at the isomerization reaction step, 3.6 parts iS fofmed at the lsomeflzatlon Sie? 1U all amount 0f uP of the mixture containing 2,7-DMN at a high content to 1.8 mole percent based on the isomerizanon product, were removed from the system. accumulation of 2,7-DMN can be prevented in the isomer- TABLE 21 Compositions (mole percent) Crystals Crystals clelreiizteirllltergterg separated at methanol Starting Isomerl- 0 C. from mother liquor DMN zation isomerization used for mixture product product purification (I) (I (In) (IV) When the methanol liquor from which 3.6 parts of ization cycle. Therefore, the yield of 2,6-DMN can be crystals had been separated by filtration was distilled to increased. remove methanol therefrom, 6.5 parts of a DMN mixture EXAMPLE 2 were obtained. Then the DMN mixture was combined with a DMN fraction obtained by distilling the mother in eralln; xtr amleqigolgecgllert hquor rema'lmng after separatlon of tlle bove crystals 55 3.0 to a reaction tube packed with a H -type mordenite formed at 0 ,C' As a resllll 63'0 Parts o, a MN mlxture catalyst, and the 1somer1zat1on was carried out at 290 of the following Composltlon were obtamedi C. to yield a reaction product having a composition indicated in Table 3.

Mole Percent 100 parts of the so obtained isomerization product were 2,6'DMN 7-1 cooled at 0 C. (lower by 7 C. than the two-component 1:6DMN 58-1 60 eutectic mixture-precipitating temperature of 7 C.) to 1,5-DMN 12-5 precipitate crystals. As a result 44.0 parts of a DMN 27'DMN 9-8 mixture having a composition III indicated in Table 3 17'DMN 12-2 were obtained. The crystals were incorporated into 80 Low'bolhng'polm and hlgh'bollmg-Polnt bYPfOd' parts of methanol, agitated at 15 C. to effect washing,

cts 0-3 65 and separated from the washing liquor by filtration. AS a result 36.3 parts of 2,6-DMN of 99.0% purity were In the above separation procedures, 29.5 parts of Obtained, t2, 5*PM2N701g1?/`I9I1`?7; Plllitl, 3.6tPatrS f4th1e mituffelgg' 0 Tehmther liquor from which 1crc/stals pgeciitaitled at ammg aa g conen an parso 1 a eense arate wascooe at 1 ower boiling-point and low-boiling-point by-products, totally by 3 C. than tlpie three-component eutectic mixture- 37.0 parts of these substances, were removed from the precipitating*temperature of -7 C.) to precipitate 4.3 isomerization cycle. The amount corresponding to this parts of 2,7-DMN-containing crystals having a composiamount removed, namely 37.0 parts, of 1,5-DMN was tion indicated in Table 3. The crystals were removed from combined with the above mixture to be recycled to the the isomerization cycle so as to prevent accumulation of isomerization reaction. As a result parts of the start 2,7-DMN at the isomerization step.

TABLE 3 Composition (mole percent) Crystals obtained Cr stals mgirhcetrxiiirlor Starting Isomerlseparated at remaining DMN zation 0 C. from aiter separation mixture product isomerization of at (I) (II) product (III) -10C.(IV)

2,6-DMN 4.2 41.4 26.7 20.4 1,6-DMN 39.2 39.7 6.0 6.0 1,5-DMN 50.0 8.6 5.8 47.2 2,7-DMN 2.9 4.0 0.6 18.8 Z'I-Dni i t.- 3.7 3.8 0.5 0.6 tmgi {y-odus 0.0 2.5 0.4 0.4

The mother liquor remaining after separation of crys- Mole percent tals formed by cooling at 10 C. was combined with 2,6-DMN 5.4 the washing liquor remaining after iiltration of 2,6'DMN 20 1,6-DMN 72.9 of 99.0% purity, and methanol was distilled therefrom, 1,5-DMN 8,8 to thereby yield 56.5 parts of a DMN fraction having the 2,7-.DMN 5,7 following composition 1,7-DMN 7,2 Mole pengu; This DMN fraction Iwas blended with 47.8 parts of 2,6DMN 693 25 1,5DMN to yield 100 parts of a starting DMN mixture 1,6-DMN 11'4 having the following composition: 15-DMN '5'2 Mole percent 2,7-DMN 6.6 2,6 DMN 2.8 1,7-DMN 1,6DMN 38.1 When 1,5-DMN was incorporated into the above fraction in an amount corresponding to the amount of the ITDMN 3'8 substances removed from the system, namely 43.5 parts, 100 parts of the starting DMN mixture were obtained. What we clarm 1s: h The overall yield of 2,6-D1MN attained in this example 35 1. In the preparatron of 2,6-d1methyl naphthalene by was 83.5%.

EXAMPLE 3 When 100 parts of the isomerization product obtained in Example 2 were cooled at 8 C. (higher by 1 C. than the two-component eutectic mixture-precipitating temperature of 7 C.), crystals were precipitated, and the precipitated crystals were separated to yield 40.6 parts of crystals having a composition indicated in Table 4. The crystals were washed with methanol to yield 36.6 parts of 2,6-DMN of 99.0% purity.

'I'he mother liquor from which the crystals precipitated at 8 C. had been separated was then cooled to 10 C. to precipitate crystals. As a result 7.6 parts of 2,7-DMN- containing crystals having a composition (LII) indicated in Table 4 were recovered.

TABLE 4 Composition (mole percent) Crystals separated from Crystals sepamother hquor The mother liquor from which crystals precipitated at -10 C. had been separated was combined with the mother liquor remaining after separation of 2,6-DMN of 99.0% purity, and the washing solvent, i.e., methanol was removed from the mixed liquor.. The residue was fractionated to yield 52.2 parts of a DMN fraction having the following composition:

feeding 1,5-dimethyl naphthalene, 1,6-dimethyl naphthalene, a mixture thereof, or a mixture containing such dimethyl naphthalene as the main component to an isomerization reaction zone, isomerizing said dimethyl naphthalene in the presence of an isomerization catalyst, cooling the resulting isomerization reaction product to crystallize 2,-6-dimethyl naphthalene, and separating and recovering the same, the improvement wherein an eutectic 2,7-dimethyl naphthalene mixture is removed at a stage after isomerization and before the mother liquor remaining after the recovery of 2,6-dimethyl naphthalene from the isomerization reaction product is recycled to the isomerization reaction zone, by cooling the mother liquor to or lower than a temperature at which the eutectic mixture of 2,7-dimeth`yl naphthalene crystallizes.

2. The process of claim 1, wherein monoand trimethyl naphthalenes are distilled off at a stage before the mother liquor remaining after the recovery of 2,6-dirnethyl naphthalene from the isomerization reaction product is recycled to the isomerization reaction zone.

3. The process of claim 1, wherein the removal of 2,7- dimethyl naphthalene or eutectic mixture thereof is effected by a combination of steps including cooling the isomerization reaction product at a temperature at which 2,6-dimethyl naphthalene is allowed to crystallize or at ya temperature lowerl than said temperature but higher than the temperature at which 2,7-dimethyl naphthalene or eutectic mixture thereof is not allowed to precipitate, separating and recovering the crystallized 2,6-dimethyl naphthalene, cooling the remaining mother liquor at a temperature at which 2J-dimethyl naphthalene or eutectic mixture thereof is allowed to crystallize or at a temperature lower than said temperature, and separating and removing the crystallized product.

4. The process of claim 3, wherein the separated crystalized 2,6-dimethyl naphthalene is treated with an organic solvent having a low dissolving property for 2,6-dimethy1 naphthalene but having a high dissolving property for 2,7-dimethyl naphthaylene or eutectic mixture thereof, to thereby purify the 2,-'6-dirnethyl naphthalene, and then the purified 2,6-dimethyl naphthalene is recovered.

1 1 12 5. The process of claim 4, wherein the purificaton 3,155,738 11/ 1964 Suld 260-668 F solvent is removed from the mother liquor remaining after 3,665,043 5/ 1972 Davis et al 42604-674 N the recovery of the purified 2,6-dimethyl naphthalene, and 3,541,175 11/ 1970 Hedge 2611-674 N the residue is recycled to the isomerization reaction zone. v3,590,091 l6/ 1971 Skarada et al. 260-674 N References Cited 5 CURTIS R. DAVIS, Primary Examiner UNITED STATES PATENTS U S CL X'R.

3,109,036 10/196-3 Suld et al 260-668 A 260-668 A, 674 N 

